The study, funded by the California Air Resources Board (ARB) and led by Dr. Veerabhadran Ramanathan of the Scripps Institution of Oceanography at the University of California, San Diego, estimates that reductions in black carbon as a result of clean air regulations were equivalent to reducing carbon dioxide emissions in California by 21 million metric tons annually or taking more than 4 million cars off California roads every year. The reductions occurred during a time when diesel fuel consumption increased by about a factor of five.

If California’s efforts in reducing black carbon can be replicated globally, we can slow down global warming in the coming decades by about 15 percent, in addition to protecting people’s lives. It is a win-win solution if we also mitigate carbon dioxide emissions simultaneously.

—Dr. V. Ramanathan

Black carbon—tiny soot particles released into the atmosphere by burning fuels—has been linked to adverse health and environmental impacts through decades of scientific research. It is also one of the major short-lived contributors to climate change. The major sources of black carbon in California are diesel-burning mobile sources, residential wood burning in fireplaces and heaters, agricultural burning and wildfires.

Black carbon has the effect of warming the atmosphere because it is effective at absorbing sunlight. However, it is emitted together with a range of other particle pollutants, including organic carbon, sulfur and other chemicals, some of which have a cooling effect, typically by reflecting sunlight. Reducing diesel emissions can therefore lead to a reduction of both warming and cooling particles.

The report, however, is the first to confirm, based on both observations and computer modeling, that the warming effect of black carbon dominates, overwhelming any cooling effect of other pollutants. This confirms the positive impact reducing diesel emissions has on fighting climate change.

...for regions like California, where mitigation policies have historically targeted primarily fossil fuel sources leading to a large decrease in atmospheric BC, the climate benefits of direct forcing reduction has masked the net warming due to greenhouse gases by a measurable fraction (estimated to be 5% of the CO₂ warming potential). This climate benefit dates back to at least the 1960s, and is currently ongoing. Brown carbon, emitted from residential wood burning is found to be another attractive target for policy makers seeking to combat anthropogenic climate change.

—“Black Carbon and Regional Climate of California”

The 3-year-study, titled “Black Carbon and Regional Climate of California,” was conducted by UC San Diego and the US Department of Energy’s Lawrence Berkeley National Laboratory (LBNL) and Pacific Northwest National Laboratory (PNNL). It is the first comprehensive regional assessment of the climate impact of black carbon on California. In conducting the study, scientists used computer models and air pollution data collected by aircraft, satellite and ground monitors.

According to co-author Dr. Tom Kirchstetter of LBNL, black carbon levels have decreased by about 90% over a 45-year period, beginning with the establishment of CARB in 1967, mostly as a result of state regulations for diesel engine emissions. Researchers found the state’s efforts to reduce diesel emissions to have lessened the impact of global warming on California, supporting earlier theoretical computer modeling by Dr. Mark Jacobson of Stanford University that reducing black carbon from diesel combustion is a potent climate cooler.

The study took a conservative approach in examining the impact black carbon has on the Golden State. Researchers considered emissions only from diesel-powered trucks and buses, and off-road diesel equipment and vehicles to estimate the equivalent reduction of carbon dioxide.

When all sources of black carbon emissions from diesel fuel combustion are considered, including farming and construction equipment, trains and ships, the reduction in carbon dioxide emissions can be as high as 50 million metric tons per year over the past 20 years. That’s roughly equal to a 13% reduction in the total annual carbon dioxide emissions in California.

The study found that co-emitted pollutants such as sulfates and organic carbon did not decrease at the same time as the black carbon. Many of these co-emitted pollutants reflect light back into the atmosphere causing cooling that can offset some of the warming caused by black carbon. These results support a growing body of evidence that mitigation of black carbon emissions, particularly from diesel engines, can provide fast mitigation of global warming.

The study also found that brown carbon—a type of organic carbon that is typically ignored in climate models—is also a potent warming agent, offsetting up to 60 to 90% of the cooling caused by other lighter organic carbons. Brown carbon is emitted primarily from sources such as forest fires and residential wood burning, which previous studies believed to have negligible climate effect, or even a cooling effect. The results from the California study indicate that reducing emissions from these sources may also provide a benefit to climate mitigation.

In addition, they study found that black carbon particles increased the number of drops of water in clouds, while decreasing the size of those drops, a condition that can reduce or delay rain.

Resources

• Black Carbon and the Regional Climate of California. Report to the California Air Resources Board Contract 08-323

• Bahadur, R., Praveen, P. S., Xu, Y., Ramanathan, V. (2012) Solar absorption by elemental and brown carbon determined from spectral observations. Proceedings of the National Academy of Sciences 109.43: 17366-17371 doi: 10.1073/pnas.1205910109